Gamma Oscillations and Photosensitive Epilepsy

Gamma oscillations (GO) have a frequency between 30 and 100 Hz and a bandwidth of about 30 Hz and have been related to neuronal synchrony involved both in generation of seizures and cognitive processes. Human and animal studies suggest that a circuitry including excitatory and inhibitory neurons is a potential generator of GO, but they are also highly dependent on the characteristics of the visual stimulus. These stimuli that are effective in eliciting photoparoxysmal responses (PPR) are also able to drive narrowband GO (30-80 Hz) in visual cortex PPRs elicited by intermittent photic stimulation in photosensitive patients were preceded by a prominent gamma component with increase in intra- and interhemispheric mean coherence values of the gamma band. Gratings with spatial frequency and color that trigger more frequently PPRs were also found to elicit more prominent GO. Modeling studies have shown that such a circuit can generate stimulus varying gamma oscillations and epileptic activity when it acts as an inhibition-stabilized network.